"We were surprised by the negativity of the commissioned review, and found it misleading and not an accurate reflection of available evidence. E-cigarettes are new and we certainly don't yet have all the answers as to their long-term health impact, but what we do know is that they are much safer than cigarettes, which kill over 6 million people a year worldwide," said Ann McNeill, lead author from the National Addiction Centre at King's College London, according to a news release.

Some health experts believe that e-cigarettes can act as "gateway drugs" to addiction and substance abuse. A recent study had found that exposure to nicotine led to increase in cocaine dependence in mouse models.

E-cigarettes use battery-powered cartridges that produce a nicotine-laced vapour. Their popularity, especially among young adults, has soared in the past two years.

The critique of the WHO report by McNeill and her co-authors has been published in the journal Addiction. The report suggests that e-cigarettes expose bystanders to toxic chemicals and that these devices prevent smoking cessation. According to McNeill and colleagues, there is currently no evidence to back these claims.

"E-cigarette use has been a consumer led revolution, the speed at which these products have developed and evolved shows just how much smokers are ready to adopt harm-reduction products. The use of e-cigarettes could save millions of lives during this century, and have the most important public health impact in the history of tobacco use," said Dr Jacques le Houezec, co-author and consultant in Public Health and Tobacco dependence in France.